The present invention relates to electromagnetic vibration generators, sometimes known in the art as shakers, which are employed for the vibration testing of components, apparatus and equipment in numerous branches of industry and research.
Such electromagnetic vibration generators basically consist of an armature which is suspended from a rigid body or stator and having a coil carried by the armature located in an air gap in the stator. A D.C. magnetic field is generated across the air gap either by permanent magnets or electromagnets and when an alternating current is fed through the armature coil, the armature is caused to vibrate along its axis at the frequency of the applied alternating current. The armature is mounted on bearings and has peripheral suspension members which center the armature in the air gap and allow free movement along its axis of vibration but which impose a high stiffness to any lateral movement of the armature normal to its axis of vibration. An article to be vibration tested may be placed directly on top of the armature or on a work table carried by the armature when the vibration testing is to be carried out in the vertical mode, or the article to be tested may be placed on a horizontal slip table coupled to the armature when the vibration testing is to be carried out in the horizontal mode, as is well known in the art.
Although as mentioned above, armature suspension systems are designed to provide a high degree of stiffness to any lateral movement of the armature, existing systems do not necessarily provide a high restraint to torsional forces tending to cause turning movement of the armature about its axis of vibration. Such torsional forces may be induced partly due to the helical form of the armature coil and/or by resonances induced in the armature in relation to its suspension system at one or more particular frequencies throughout the range of frequencies employed for the vibration testing. This can in turn give rise to problems during the vibration testing of certain types of components, such as gyroscopes, which are sensitive to spurious rotational forces or movements.